This invention relates to the provision of polarisation state insensitive wavelength multiplexing 2.times.2 tapered fused fibre couplers.
It is known that mutual coupling between a pair of identical optical fibres can be achieved by arranging them in side-by-side contact, heating them to produce adhesion, and then stretching them, while hot, to produce localised plastic flow in order to provide a region of reduced cross-section where the evanescent fields of the two fibres are enlarged by an amount providing substantial overlap of those evanscent fields. This overlapping of the evanscent fields produces a mutual coupling of the two fibres, and thus there is produced a 2.times.2 tapered fused fibre coupler. The strength of the coupling depends upon the amount of overlap, which is both mode dependent and wavelength dependent, and depends also upon the length and shape of the coupling region. If the two fibres are single mode fibres, the stretching may be controlled to produce a wavelength multiplexing 2.times.2 tapered fused fibre coupler having, in principle, the property that light of one specified wavelength, hereinafter referred to as the minimum power transfer wavelength, launched into one end of one of the fibres will emerge substantially exclusively from the other end of that same fibre, while light of another specified wavelength, hereinafter referred to as the maximum power transfer wavelength, launched into the same one end of the same one of the fibres will emerge substantially exclusively from the other end of the other fibre. In practice, because the coupling region of reduced cross-section does not possess circular symmetry, the coupling coefficient for light of any given wavelength that is plane polarised in the plane that contains the axes of the two fibre cores is not exactly the same as that for the same wavelength plane polarised in the perpendicular plane. In practice therefore the coupler exhibits birefringence whose priciple planes are those two planes respectively in, and perpendicular to, the plane that contains the axes of the two fibre cores, and in consequence there are different minimum and maximum power transfer wavelengths for each of the two principle planes.
A particular convenient way of making 2.times.2 tapered fused fibre couplers in a manner readily capable of producing regularly reproducible results is the progressive stretching method described in GB 2 150 703 B. This progressive stretching method uses a pair of clamps mounted on independent linear motion drives designed to move in the axial direction defined by the line joining the two clamps. A stranded pair of fibres is secured to extend in the axial direction between the two clamps, and beneath the fibres is located a burned to provide a localised hot zone. With the burner lit to heat the fibres to a temperature at which the viscosity is reduced just enough to allow plastic deformation under tension, the two clamps are caused to move at controlled rates in the same direction. The leading clamp is caused to move a controlled amount faster than the trailing clamp so as to cause plastic stretching of the fibre at a controlled rate. In this way a stretched region of slightly reduced cross-section is produced that terminates at each end in a small taper up to the full-size, unstretched, regions of stranded fibre. The proportional reduction in cross-section, the draw-down ratio, produced by a single traversal is determined by the speeds of the two drives. The length of the reduced cross-section region can be varied independently of the draw-down ratio by varying the duration of the traversal. Typically between five and ten traversals may be employed to effect something in the region of a forty to fifty-fold reduction in cross-sectional area. Conveniently successive traverses may be performed in alternate directions, with the leading drive of one traverse becoming the trailing drive of the next traverse. It is also generally desirable to make each successive traverse slightly shorter than the immediately preceding traverse so that a relatively smooth adiabatic taper is produced at each end of the reduced cross-section region from that size right up to the cross-section of the full-size unstretched regions.
By launching light into one end of one of the two stranded fibres, and looking at what proportion emerges from its far end in relation to that which emerges from the far end of the other fibre, the amount of coupling provided by the progressive stretching can be continuously monitored as the stretching proceeds. GB 2 150 703 B explains that an advantage of using a very small width burner is that only a small proportion of the total length of the reduced cross-section length is heated at any one time, and hence the overall coupling strength changes relatively little when the burner is turned off and the refractive index of the heated portion changes due to thermal effects. This is clearly advantageous if one wants to perform the whole progressive stretching operation in one continuous interrupted burn. If, on the other hand, one is content to approach close to the end point, and then proceed in a series of short duration bursts of the burner (synchronised with bursts of progressive stretching), then one can readily monitor the `cold` coupling strength between successive bursts, in which case it may be advantageous to use a burner of more extended width.
Another significant controllable variable of the progressive stretching method is the heat of the burner. If the burner provides a relatively low softening temperature, the two fibres do not coalesce to any great extent in the reduced cross-section couplng region, which in consequence has a cross-section approximating to a figure-of-eight. On the other hand, if the burner provides a significantly higher temperature, then surface tension effects are more pronounced, the re-entrants of the figure-of-eight are eliminated, and the cross-section approaches a more nearly round profile that exhibits reduced birefringence in comparison with the figure-of-eight profile.
A paper by I. J. Wilkinson and C. J. Rowe entitled `Close-Spaced Fused Fibre Wavelength Division Multiplexers with Very Low Polarisation Sensitivity` Electronics Letters Volume 26 pp 382-4 (1990) describes how the polarisation sensitivity (birefringence) of a wavelength multiplexing fused fibre 2.times.2 coupler can be substantially nulled-out by elastically twisting the coupler after its fabrication. For practical utility it is important to know what effect this twisting has upon the spectral positioning of the minimum and maximum power transfer wavelengths of the two principal polarisation planes of the coupler that were defined prior to the elastic twisting. Prior to this twisting, the coupler had minimum and maximum power transfer wavelengths for one of the principal planes of polarisation that may be respectively labelled as .lambda..sub.1 and .lambda..sub.2. Similarly, for the other principal polarisation plane, the corresponding minimum and maximum power transfer wavelengths may be respectively labelled .lambda..sub.3 and .lambda..sub.4. Prior to the twisting the multiplexer is, by definition, birefringent, and hence either .lambda..sub.1 .noteq..lambda..sub.3, or .lambda..sub.2 .noteq. .lambda..sub.4, or both .lambda..sub.1 .noteq..lambda..sub.3 and .lambda..sub.2 .noteq..lambda..sub.4. The elastic twisting serves to null-out the birefringence, and hence, after the twisting, the coupler exhibits the same minimum power transfer wavelength for all input states of polarisation (SOP's) which may be labelled .lambda..sub.5, and also, for any input SOP, corresponding maximum power transfer wavelength, which may be labelled .lambda..sub.6.
Wilkinson and Rowe assert that the channel spacing and passband positions `are unaffected by the twisting, and in a later paper entitled `Control of Polarisation Degradation in Fibre Amplifier WDM's` Electronic Letters Volume 29 No. 2 pp 214-5 (1993), Wilkinson specifically asserts that `twisting has no effect whatever on channel spacing or the wavelengths of maximum or minimum splitting`. These assertions can be alternatively stated as .lambda..sub.1 =.lambda..sub.3 =.lambda..sub.5 and .lambda..sub.2 =.lambda..sub.4 =.lambda..sub.6" but this equality relationship is inconsistent with the inequality relationship developed above. It is clear therefore that, whatever Wilkinson and Rowe may say to the contrary, it is axiomatic that, if twisting removes polarisation sensitivity, this must inevitably involve some changes in at least some of the wavelengths of maximum and minimum power transfer between the fibres. In the particular instance described in the Wilkinson and Rowe paper, the upper wavelength end of the experimental trace of FIG. 3a passes through a final maximum at a first wavelength before ending in a minimum at a second, somewhat greater, wavelength. These two wavelengths are the maximum and the minimum power transfer wavelengths for some unspecified SOP applied to the coupler before it has been twisted, and hence while it still retains its polarisation sensitivity (birefringence). FIG. 3b shows that, after twisting, the roles of these two wavelengths have been interchanged, with the wavelength that was previously the maximum power transfer wavelength, now becoming the minimum power transfer wavelength, while the wavelength, that was previously the minimum power transfer wavelength now becoming the maximum power transfer wavelength. This interchange is specifically confirmed by the later Wilkinson paper, which specifically acknowledges that `the maxima and minima swap output ports`. This swapping of the roles of the output ports means that, far from staying stationary as suggested by Wilkinson and Rowe, their example demonstrates a change in coupling strength produced by the twisting that serves to produce a wavelength shift equal in magnitude to the full wavelength separation of the two channels of their multiplexer.
Such wavelength shift produced by twisting is described by T. A. Birks in a paper entitled, `Twist-Induced Tuning in Tapered Fiber Couplers`, Applied Optics Volume 28 pp 4226-33 (1989), who employs the effect for achieving wavelength tuning. The effect is also referred to in a paper by N. M. O'Sullivan T. A. Birks and C. D. Hussey in a paper entitled, `Control of Polarisation Degradation in Fibre Amplifier WDMs`, Electronics Letters Volume 28 pp 1616-8, and again by these three authors in a later `Reply`, Electronics Letters Volume 29 pp 215, to the previously referenced later Wilkinson paper. In that reply the three authors state that they have found that twist tuning can increase the channel wavelengths of 1480/1550 nm WDM's by up to 30 nm.
For many applications the precise spectral position of the minimum and maximum power transfer wavelengths of a wavelength multiplexing coupler is of critical importance. Sometimes the positioning of one is of greater importance than that of the other. For instance, in the case of a multiplexing coupler employed in an optically pumped amplifier, the registration of one of the power transfer wavelengths with the wavelength of the optical pump power may be of less importance than the registration of the other power transfer wavelength with the wavelength of the signal to be amplified. This can be because the optical pump power source is readily tuneable, or because wastage of pump power through mis-registry is of less consequence than wastage of signal power.
During the manufacture of a poliarisation sensitive 2.times.2 fused fibre wavelength multiplexing coupler, the progress of the manufacture can be continuously monitored in a way that enables termination at a particular moment providing a relatively high level of precision in the spectral positioning of the minimum and maximum power transfer wavelengths for one of the principal planes of polarisation of the coupler, but if subsequent elastic twisting is going to produce a significant wavelength shift, the spectral positioning of the corresponding power transfer wavelengths of the polarisation sensitive nulled coupler may be incapable of being foretold with anywhere near comparable precision because of an inability to foretell with certainty the precise magnitude of that shift.